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Delete Realm Objects - Kotlin SDK

On this page

  • Delete Operations
  • Related Objects and References
  • Delete Realm Objects
  • Delete a Single Object
  • Delete Multiple Objects
  • Delete All Objects of a Type
  • Delete All Objects in a Realm
  • Delete Related Objects
  • Delete an Object and Its Related Objects
  • Delete an Embedded Object
  • Remove Elements from Collections
  • Remove Elements from a RealmList
  • Remove Elements from a RealmSet
  • Remove Dictionary Keys/Values
  • Delete RealmAny (Mixed) Property Value

This page describes how to delete objects from a local or synced realm using the Kotlin SDK.

You can choose to delete a single object, multiple objects, or all objects from the realm. After you delete an object, you can no longer access or modify it. If you try to use a deleted object, Realm throws an error.

Deleting objects from a realm does not delete the realm file or affect the realm schema. It only deletes the object instance from the realm. If you want to delete the realm file itself, refer to Delete a Realm - Kotlin SDK.

Note

Write to a Synced Realm

The syntax to delete an object from a realm is the same for a local or a synced realm. However, there are additional considerations that determine whether the delete operation in a synced realm is successful. For more information, refer to Write Data to a Synced Realm - Kotlin SDK.

All operations that modify a realm - including delete operations - must be performed inside of a write transaction. Write transactions are passed to the realm's write() or writeBlocking() method. Within this callback, you can access a MutableRealm instance and then delete objects within the realm. For more information on write transactions and how Realm handles them, refer to Write Transactions.

You can only delete live objects, which are only accessible inside of a write transaction. You can convert a frozen object to a live object in a transaction with mutableRealm.findLatest().

Example

Convert Frozen Object Before Deleting

val frozenFrog = realm.query<Frog>("name == $0", "Kermit").find().firstOrNull()
// Open a write transaction
realm.writeBlocking {
// Get the live frog object with findLatest(), then delete it
if (frozenFrog != null) {
findLatest(frozenFrog)
?.also { delete(it) }
}
}

When you delete an object that has a relationship property with another object, Realm does not automatically delete the instance of the related object. Instead, Realm only deletes the reference to the other object. The referenced object remains in the realm, but it can no longer be queried through the parent property.

The only exception is if the related object is embedded. When you delete an object that has a relationship with an EmbeddedRealmObject, Realm automatically deletes the embedded object in a cascading delete. For more information, refer to the Delete an Embedded Object section on this page.

If you want to delete any related objects when you delete a parent object, we recommend performing a chaining delete. A chaining delete consists of manually deleting dependent objects by iterating through the dependencies and deleting them before deleting the parent object. For more information on chaining deletes, refer to the Delete an Object and Its Related Objects section on this page.

If you do not delete the related objects yourself, they remain orphaned in your realm. Whether or not this is a problem depends on your application's needs.

To delete specific objects from a realm:

  1. Open a write transaction with realm.write() or realm.writeBlocking().

  2. Get the live objects by querying the transaction's mutable realm for the objects that you want to delete using query():

    1. Specify the object type as a type parameter passed to query().

    2. (Optional) Filter the set of returned objects by specifying a query. If you don't include a query filter, you return all objects of the specified type. For more information on querying with the Kotlin SDK, refer to Read Realm Objects - Kotlin SDK.

    Important

    Objects Must Be Live

    You can only delete live objects. If your query occurs outside of the write transaction, you must convert the frozen objects to live objects in the transaction with mutableRealm.findLatest().

  3. Pass the set of RealmResults returned by the query to mutableRealm.delete().

  4. The specified objects are deleted from the realm and can no longer be accessed or modified. If you try to use a deleted object, Realm throws an error.

    If any deleted objects had a relationship with another object, Realm only deletes the reference to the other object. The referenced object remains in the realm, but it can no longer be queried through the deleted parent property. Refer to the Delete an Object and Its Related Objects section for more information.

Tip

You can check whether an object is still valid to use by calling isValid(). Deleted objects return false.

To delete a single RealmObject object, query for the object type using a filter that returns the specific object that you want to delete.

Tip

Use Unique Identifying Information

We recommend filtering with unique identifying information such as a primary key value to ensure your query returns the correct object.

In the following example, we query for a Frog object with a specific primary key, and then pass the returned object to mutableRealm.delete() to delete it from the realm:

// Open a write transaction
realm.write {
// Query the Frog type and filter by primary key value
val frogToDelete: Frog = query<Frog>("_id == $0", PRIMARY_KEY_VALUE).find().first()
// Pass the query results to delete()
delete(frogToDelete)
}

To delete multiple objects at the same time, pass the object type to query() and specify a query that returns all objects that you want to delete.

In the following example, we query for the first three Frog objects whose species is "bullfrog", and then pass the results to mutableRealm.delete() to delete them from the realm:

// Open a write transaction
realm.write {
// Query by species and limit to 3 results
val bullfrogsToDelete: RealmResults<Frog> = query<Frog>("species == 'bullfrog' LIMIT(3)").find()
// Pass the query results to delete()
delete(bullfrogsToDelete)
}

To delete all objects of a specific type from a realm at the same time, pass the object type to query() and leave the query filter empty to return all objects of that type.

In the following example, we query for all Frog objects, and then pass the results to mutableRealm.delete() to delete them all from the realm:

// Open a write transaction
realm.write {
// Query Frog type with no filter to return all frog objects
val frogsLeftInTheRealm = query<Frog>().find()
// Pass the query results to delete()
delete(frogsLeftInTheRealm)
}

The Kotlin SDK lets you delete all managed objects of all types, which is useful for quickly clearing out your realm while prototyping. This does not affect the realm schema or any objects that are not managed by the realm.

To delete all objects from the realm at the same time, call mutableRealm.deleteAll(). This deletes all objects of all types.

In the following example, we delete all objects from the realm with deleteAll():

// Open a write transaction
realm.write {
// Delete all objects from the realm
deleteAll()
}

Tip

Use deleteAll() in Development

The deleteAll() method is useful for quickly clearing out your realm during development. For example, instead of writing a migration to update objects to a new schema, it may be faster to delete all, and then re-generate the objects with the app itself.

Deleting a parent object does not automatically delete any objects that are related to it unless the related object is embedded. Instead, Realm only deletes the reference to the related object.

In the following example, we have a Frog object with a list of Pond objects. After we delete the Frog object, we confirm that all Pond objects still remain in the realm:

// Open a write transaction
realm.write {
// Query for the parent frog object with ponds
val parentObject = query<Frog>("_id == $0", PRIMARY_KEY_VALUE).find().first()
assertEquals(2, parentObject.favoritePonds.size)
// Delete the frog and all references to ponds
delete(parentObject)
// Confirm pond objects are still in the realm
val ponds = query<Pond>().find()
assertEquals(2, ponds.size)
}

To delete related objects when you delete a parent object, you must manually delete the related objects yourself. We recommend chaining deletes: first query for the parent object that you want to delete, then iterate through the parent object's relationships and delete each related object. Finally, delete the parent object itself.

In the following example, we query for a Frog object named "Kermit", then iterate through the object's favoritePonds property and delete each Pond object. Then, we delete the Frog object itself:

realm.write {
// Query for the parent frog object with ponds
val frog = query<Frog>("name == $0", "Kermit").find().first()
val ponds = frog.favoritePonds
// Iterate over the list and delete each pond object
if (ponds.isNotEmpty()) {
ponds.forEach { pond ->
delete(pond)
}
}
// Delete the parent frog object
val frogToDelete = findLatest(frog)
if (frogToDelete != null) {
delete(frogToDelete)
}
}

Warning

Realm Uses Cascading Deletes for Embedded Objects

When you delete a Realm object, Realm automatically deletes any embedded objects referenced by that object. If you want the referenced objects to persist after the deletion of the parent object, use a regular Realm object with a to-one relationship instead.

You can delete an embedded object through the parent object in a cascading delete or by deleting the embedded object directly.

  • To delete the embedded object through the parent object, fetch and delete the parent object. Realm automatically deletes all of its embedded objects from the realm.

  • To delete an embedded object instance directly:

    • Fetch and delete a specific embedded object.

    • Clear the parent's reference to the embedded object, which also deletes the embedded object instance.

In the following example, we have a Business object with a list of embedded EmbeddedAddress objects. We query for and delete the Business object, which automatically deletes all of its embedded EmbeddedAddress objects:

// Delete the parent object
realm.write {
val businessToDelete = query<Business>("name == $0", "Big Frog Corp.").find().first()
// Delete the parent object (deletes all embedded objects)
delete(businessToDelete)
}

In the following example, we have Contact objects with embedded EmbeddedAddress objects. We delete an EmbeddedAddress object directly and delete another through the parent object:

// Delete an embedded object directly
realm.write {
val addressToDelete = query<EmbeddedAddress>("street == $0", "456 Lily Pad Ln").find().first()
// Delete the embedded object (nullifies the parent property)
delete(addressToDelete)
}
// Delete an embedded object through the parent
realm.write {
val propertyToClear = query<Contact>("name == $0", "Kermit").find().first()
// Clear the parent property (deletes the embedded object instance)
propertyToClear.address = null
}

Tip

Get Embedded Object's Parent

You can get the unique parent of an embedded object using parent().

Realm collection instances that contain objects only store references to those objects. You can remove one or more referenced objects from a collection without deleting the objects themselves. The objects that you remove from a collection remain in the realm until you manually delete them. Alternatively, deleting a Realm object from a realm also deletes that object from any collection instances that contain the object.

You can remove one or more elements in a single transaction from a RealmList:

  • To remove one element from the list, pass the element to list.remove().

  • To remove one element at a specified index in the list, pass the index to list.removeAt().

  • To remove multiple elements from the list, pass the elements to list.removeAll().

You can also remove all list elements at once by calling list.clear().

In the following example, we have a Forest object with a list of Pond objects. We remove the list elements in a series of operations until the list is empty:

// Open a write transaction
realm.write {
// Query for the parent forest object
val forest = query<Forest>("name == $0", "Hundred Acre Wood").find().first()
val forestPonds = forest.nearbyPonds
assertEquals(5, forestPonds.size)
// Remove the first pond in the list
val removeFirstPond = forestPonds.first()
forestPonds.remove(removeFirstPond)
assertEquals(4, forestPonds.size)
// Remove the pond at index 2 in the list
forestPonds.removeAt(2)
assertEquals(3, forestPonds.size)
// Remove the remaining three ponds in the list
forestPonds.removeAll(forestPonds)
assertEquals(0, forestPonds.size)
}

In the following example, we have a Forest object with a list of Pond objects. We remove all list elements with the list.clear() method:

// Open a write transaction
realm.write {
val forest = query<Forest>("name == $0", "Hundred Acre Wood").find().first()
val forestPonds = forest.nearbyPonds
assertEquals(5, forestPonds.size)
// Clear all ponds from the list
forestPonds.clear()
assertEquals(0, forestPonds.size)
}

You can remove one or more elements in a single transaction from a RealmSet:

  • To remove one element from the set, pass the element you want to delete to set.remove().

  • To remove multiple elements from the set, pass the elements you want to delete to set.removeAll().

You can also remove all set elements at once by calling set.clear().

In the following example, we have a Frog object with a set of Snack objects. We remove the set elements in a series of operations until the set is empty:

// Open a write transaction
realm.write {
// Query for the parent frog object
val myFrog = query<RealmSet_Frog>("name == $0", "Kermit").find().first()
val snackSet = myFrog.favoriteSnacks
assertEquals(3, snackSet.size)
// Remove one snack from the set
snackSet.remove(snackSet.first { it.name == "Flies" })
assertEquals(2, snackSet.size)
// Remove the remaining two snacks from the set
val allSnacks = findLatest(myFrog)!!.favoriteSnacks
snackSet.removeAll(allSnacks)
assertEquals(0, snackSet.size)
}

In the following example, we have a Frog object with a set of Snack objects. We remove all set elements with the set.clear() method:

realm.write {
val myFrog = realm.query<RealmSet_Frog>("name == $0", "Kermit").find().first()
val snackSet = findLatest(myFrog)!!.favoriteSnacks
assertEquals(3, snackSet.size)
// Clear all snacks from the set
snackSet.clear()
assertEquals(0, snackSet.size)
}

You can remove RealmDictionary entries in a few ways:

  • To remove the value but keep the key, set the key to null (the dictionary's value must be nullable)

  • To remove the key and the value, pass the key to remove()

You can also remove all keys and values by calling clear().

In the following example, we have a Frog object with a dictionary of String values. We remove the dictionary elements in a series of operations until the dictionary is empty:

// Find frogs who have forests with favorite ponds
val thisFrog = realm.query<Frog>("favoritePondsByForest.@count > 1").find().first()
// Set an optional value for a key to null if the key exists
if (thisFrog.favoritePondsByForest.containsKey("Hundred Acre Wood")) {
realm.write {
val mutableFrog = findLatest(thisFrog)
if (mutableFrog != null) {
mutableFrog.favoritePondsByForest["Hundred Acre Wood"] = null
}
}
}
realm.write {
// Remove a key and its value
findLatest(thisFrog)?.favoritePondsByForest?.remove("Lothlorien")
// Remove all keys and values
findLatest(thisFrog)?.favoritePondsByForest?.clear()
assertTrue(thisFrogUpdated.favoritePondsByForest.isEmpty())
}

Although RealmAny instances cannot store null values, you can delete a RealmAny property value by assigning null directly to the property. For more information on the RealmAny data type, refer to RealmAny (Mixed).

In the following example, we have a Frog object with a list of RealmAny properties, and we clear the first RealmAny property value:

realm.write {
val frog = query<Frog>().find().first()
frog.favoriteThings[0] = null
}
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